An adequate theory of language acquisition must take account of both the internal and environmental constraints on the acquisition process. Recent work on the syntactic environment alone indicates that it does not function as a model from which to induce language directly. But, the communicative environment consists of other sources of information that can act as constraints on the child's possible hypotheses about language. The child's cognitive capacities and strategies for dealing with interactions further constrain interpretation of language. Previous research has described aspects of the communicative environment and has given evidence for early action-based strategies. The proposed research includes (1) studies examining both the normal and the language-impaired child's ability to process and integrate various kinds of contextual information relevant to understanding messages; (2) intervention studies using selected linguistic and non-linguistic aspects of interactions to assess their role in the development of both syntactic and conversational skills. The studies on normal children, 1 year to 3 years, are concerned with the importance of gesture for understanding, the relation of gesture to word order as determiners of responding, and the development of "what" as a stop-action marker in varied contexts. The intervention studies with normal 2-year-olds test the effect of input as a function of its relation to joint activity and sequencing in the conversation. The studies are unique in that they call for analyses of maternal input as well as intervention, and for post-testing for generalization as well as repeated post-testing. The preliminary work on language-impaired children, aged 3 to 7, examines their ability to use context to assign interpretations to ambiguous sentences. The objective is to develop eventually a procedure for identifying children who have problems processing and integrating linguistic and non-linguistic sources of information. These studies are novel attempts to provide evidence for mechanisms of language acquisition that depend on both internal and external constraint.